Method of burying an object in the soil

ABSTRACT

A method of burying objects, for example pipelines and cables in the bottom of the sea includes loosening the soil, laying the object in the loosened soil and compacting the soil placed on top of the object laid by means of vibrators.

United States Patent Jansz [54] METHOD OF BURYING AN OBJECT IN THE SOILInventor: Joost Werner Jansz, Rijswijk, Netherlands Assignee:Nederlandse Maatschappij voor Werken Buitengaats (Netherlands OffshoreCompany) N.V., The Hague, Netherlands Filed: Sept. 3, 1970 Appl. No.:69,236

US. Cl ..6l/72.4, 61/35 Int. Cl. ..Fl6l 1/00, E02f 5/12 Field ofSearch..6l/72.4, 72.1, 72.3, 36

[ 1 May23, 1972 [56] References Cited UNITED STATES PATENTS 3,195,3147/1950 Degen ..61/50 3,540,226 II/ 1970 Sherrod .61/72.4

FOREIGN PATENTS 0R APPLICATIONS 1,428,155 0/1966 France ..6l/72.4

Primary E.xaminerJacob Shapiro Attorney-Holcombe, Wethen'll & BriseboisABSTRACT A method of burying objects, for example pipelines and cablesin the bottom of the sea includes loosening the soil, laying the objectin the loosened soil and compacting the soil placed on top of the objectlaid by means of vibrators.

3 Claims, 1 Drawing Figure METHOD OF BURYING AN OBJECT IN THE SOIL Thisinvention relates to a method of burying an object, for examplepipelines, cable or the like, in the bottom of the sea (which word isused herein to include rivers and other water areas).

Objects that are buried in soil under the so called phreatic surfacehave a tendency to float up when the specific gravity of the object isless than the specific gravity of the soil in loosened conditions.Usually an object is lowered into a ditch or trench by imposing a loadon it. This ballast prevents the object from floating up. In many casesthe load is removed as soon as the object is covered by soil, whichconsequently has a loose structure. When for some reason this loose soilwhich covers the object starts to settle and some internal flowing leadsto a spontaneous liquification, this soil will temporarily behave like aheavy liquid with a specific gravity that can exceed that of the object.The object may then float up in this heavy liquid, displacing the soilwhich immediately settles in a more dense structure than it was beforethe internal flowing occurred.

In order to prevent an object that is buried under the phreatic surfacein the soil from floating up in its surrounding soil as a result of theinternal flowing patterns, the method of this invention consists indeliberately vibrating the loose soil on top of the object in order toforce the soil to settle quickly in a more dense structure while theimposed load on top of the object is still acting on it. This ensuresthat no internal flowing can occur, and the imposed load can safely beremoved.

An example of the application of the method of this invention to buryinga pipeline in the sea bed will now be described. To bury an offshore gaspipeline which, for instance, has a specific gravity when immersed of1.3 in a sandy seabottom, a normal practice is to loosen the soil bymeans of strong jet streams which cause the sand to come into afluidized state. By the weight of the trenching machine working on thepipe, the latter will be lowered in to this fluidized sand until itreaches the unloosened sand. Due to the stiffness of the pipe, thedownward force of the trenching machine will keep the pipe in itslowered position even at some distance behind the trencher where thesand deposits over the pipe in a loose condition. By applying vibrationsto the sand in the said area, the loose sand particles are forced tosettle into a denser structure caused by its spontaneous internal flow,and the floating up of the pipe is prevented. When the trenching machinemoves forward, the pipeline left behind is buried and covered by asufficiently compact sand layer of such a dense structure that it willbe kept in position in spite of its relatively low specific gravity.

A preferred embodiment of the invention will now be described withreference to the accompanying drawing which shows a schematic sectionalview through the sea bed illustrating the laying ofa pipeline.

Referring to the drawing, there is shown a pipeline 1 which has beeninitially laid on the seabed 2 as indicated at 3. A trenching apparatus,indicated diagrammatically at 4 by dotted lines, and which may be of thetype described in copending patent applications Ser. No. 18538/69 andNo.

36650/69, has moved along a part of the pipeline in the direction of thearrow 4a and loosened the soil around the pipeline by means includingjet streams of water to form a trench-like region, thereby enabling thepart of the pipeline along which it has moved to sink through the saidregion to the level shown at 5. The loosened soil mixed with water isconveyed to the rear of the apparatus 4 and the soil is placed on top ofthe pipeline lying at the level 5. The surface of the soil being placedon top of the pipeline in the trench falls away naturally from a peaktowards the apparatus 4 and on this sloping surface behind the apparatus4 there is arranged a plurality of torpedo shaped vibrators 6, 7 and 8,of known design and suitable for underwater working, which arehydraulically operated via connecting lines 9, 10 and 11 from theapparatus 4. The connecting lines 9,10 and 11 also include means forhauling the vibrators 6, 7 and 8 behind the trenching apparatus as theapparatus 4 moves along the pipeline. it will be noted that the vibrator6, which is at the lowest level, precedes the vibrators at a higherlevel, in order to compact the soil nearest to the pipeline first. Thedegree of the compacting of the soil in the trench-like region isrepresented on the drawing and indicates that the soil at 12 is moredensely compacted than that at 13 and that at 13 is more denselycompacted than that at 14 because the soil at 12 has been compacted bythe vibrators 6, 7 and 8 and that at 13 by the vibrators 6 and 7, whilethe soil at 14 has only been compacted by the vibrator 6 passing overit. The number and spacing of the vibrators, in a row next to eachother, depends upon the width of the loosened soil. Of course, ifnecessary, more than one train of vibrators may be employed. Thevibrators may be operated by other means, for example pneumatically, andit will be understood that their power and the frequency at which theyvibrate will be chosen to ensure sufiicient compacting of the soil atthe particular speed of movement to prevent the pipeline from risingfrom the level 5.

While a particular embodiment has been described it will be understoodthat various modifications can be made without departing from the scopeof the invention, For example, in certain circumstances it is necessaryto use only one vibrator and the vibrator or vibrators may be hauled bymeans other than the trenching apparatus.

I claim:

1. A method of burying a pipeline, cable or the like in the bottom ofthe sea (as herein defined) which includes the step of loosening thesoil in a region of the sea bottom by means of a trenching apparatuswhich is movable along the pipeline, lowering the object into the regionin which the soil has been loosened and hauling one or more vibratorsattached to the trenching apparatus behind the apparatus as it movesalong the pipeline in order to vibrate and compact the soil on top ofthe pipeline.

2. A method according to claim 1 including the step of positioning thevibrators at different levels above the pipeline.

3. A method according to claim 2 wherein the vibrators at a low levelprecede those at a higher level.

1. A method of burying a pipeline, cable or the like in the bottom of the sea (as herein defined) which includes the step of loosening the soil in a region of the sea bottom by means of a trenching apparatus which is movable along the pipeline, lowering the object into the region in which the soil has been loosened and hauling one or more vibrators attached to the trenching apparatus behind the apparatus as it moves along the pipeline in order to vibrate and compact the soil on top of the pipeline.
 2. A method according to claim 1 including the step of positioning the vibrators at different levels above the pipeline.
 3. A method according to claim 2 wherein the vibrators at a low level precede those at a higher level. 